The present invention relates to an optical fiber-based device and, more particularly, to a fiber-based polarization controller utilizing optical non-linearity-induced tunable birefringence to provide polarization control.
Optical signals propagating in standard, non-polarization preserving optical fiber-based communication systems experience random changes in polarization state from one end of the fiber to the other, as a result of fiber birefringence induced by, among other factors, temperature fluctuations and physical stresses on the fiber. Random polarization changes are evidenced at the output end as polarization dependent loss (PDL) and in some instances polarization mode dispersion (PMD). In-line photonic components typically possess some inherent level of polarization-based loss. As a result, a varying state of polarization (SOP) at the input of such a device will cause random intensity fluctuations in the signal passing through the in-line device. Coherent communications systems rely on interference of the transmitted signal with a local oscillator, and maximum extinction ratios of such interference is guaranteed only when the polarization states of the two signals are identical. A signal with a randomly varying SOP will therefore cause random fluctuations in received power.
The distortions due to PMD may be alleviated by the use of an optical PMD compensator, such as disclosed in U.S. Pat. No. 5,930,414 issued to D. A. Fishman et al. on Jul. 27, 1999. A critical device for enabling a PMD compensator is a polarization controller, which is defined as a device that can alter the SOP of a lightwave signal at the input of a polarization controller to any arbitrary SOP at the output of the polarization controller. Such a polarization controller can also be used to provide for a fixed SOP of a signal entering another device exhibiting polarization dependent loss, or to provide a fixed SOP input signal at an interferometer in coherent communications systems.
Polarization controllers for fiber optic applications have been designed or demonstrated with materials whose birefringence, xcex93, can be altered by either the electro-optic response of a material such as lithium niobate (LiNbO3), or thermo-optic and elasto-optic response of amorphous materials such as silica fibers. Polarization controllers using the electro-optic effect may be realized in planar waveguides of LiNbO3. Such planar waveguide polarization controllers have been found to exhibit sufficient speed of operation (e.g., response time on the order of microseconds), but involve complex fabrication steps that make the device expensive to manufacture. In addition, planar waveguide devices have been found to be susceptible to polarization dependent loss and exhibit high insertion losses.
Thermal and mechanical stresses in silica fibers have also been used to fabricate a variety of different polarization controllers where it has been found that uniaxial stress applied on nominally circular silica fibers leads to birefringence (since a fiber under uniaxial stress has dissimilar propagation constants for the two orthogonal polarizations of propagating lightwave signals). Changing the amount of stress on the fiber changes xcex93, and a cascade of these elements oriented at different angles can form a polarization controller that is capable of transforming any input SOP to any arbitrary output SOP. Variations of this device have included fiber squeezers that allow rotation of squeezed fiber (thus allowing simultaneous changes in the angle of orientation and birefringence). Alternatively, fiber-based polarization controllers can be formed by rotating or heating cascaded segments of inherently birefringent fibers. All of these fiber-based polarization controllers have the inherent advantages of being compatible with any conventional fiber optic communication system (thus providing an in-line, low loss polarization controller). However, thermal or mechanical control is slow by nature, and the response time of such devices is typically higher than 100 microseconds. In addition, stress-induced polarization controllers are susceptible to breakage and fatigue, thus posing reliability constraints.
Thus, a need remains in the art for a polarization controller that combines the fast, reliable response achievable in electro-optic waveguide devices with the low-loss, cost-effective and fiber-compatible characteristics of existing fiber-based polarization controllers.
The need remaining in the art is addressed by the present invention, which relates to an optical fiber-based device and, more particularly, to a fiber-based polarization controller utilizing induced circular asymmetry in an optical fiber to provide polarization control.
In accordance with the present invention, a section of optical fiber is formed to exhibit spatial asymmetry in terms of its nonlinear optical response to form a device with a variable birefringence. If either the composition of the optical fiber or the intensity pattern of an applied optical pump signal is not circularly symmetric in the fiber, the fiber will become birefringent. In operation, a pump beam control signal is applied as an input to the section of optical fiber, along with an input signal of unknown (or uncontrolled) polarization. The intensity of the pump beam is used to control the magnitude of the birefringence of the asymmetric fiber and thus the state of polarization (SOP) of the signal passing therethrough. Several such devices can then be cascaded to provide arbitrary control SOP transformations.
In one embodiment of the present invention a fiber may be fabricated to incorporate a dopant (such as erbium or vanadium) that is deposited in a non-circularly symmetric manner and exhibits a doping profile that induces optical non-linearity. In an alternative embodiment, asymmetry can be generated by launching a pump beam into a spatially asymmetric mode, such as one of the LP[1,m] modes. The simultaneous introduction of an optical signal into the conventional LP[0,1] mode and the pump into the asymmetric LP[1,m] mode can be accomplished using standard wavelength division multiplexing (WDM) elements and long-period fiber gratings (LPGs).
Other and further embodiments and uses of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.